The present invention relates to communications systems and more particularly to a differentially phase modulated communication transmission system.
In the field of communications it has been the general practice to transmit digital information by periodically varying the phase of a carrier signal in accordance with the information being transmitted. If the phase of the carrier signal is varied instantaneously by, for example, balanced modulation, overtones and beat frequencies will be generated, thereby substantially increasing the bandwidth of the communications channel. However, if phase variation of the carrier signal can be accomplished gradually then the bandwidth required for the communications channel will be relatively narrow.
In the present invention a carrier frequency is phase modulated by directing it through a voltage controlled phase shifter to a load, with the voltage controlled phase shifter being controlled in response to the phase difference between the carrier frequency and the signal output of the voltage controlled phase shifter as directed through one of a plurality of fixed phase shifters, which is selected in accordance with the desired modulation.
A phase detector is used to generate a signal which is related to the phase difference between the carrier frequency and the output from the voltage controlled phase shifter as directed through the selected fixed phase shifter. This signal is applied through a low pass filter to control the voltage controlled phase shifter.
It is a primary object of this invention to provide a phase modulator adapted for use in a phase shift key (PSK) communications system.
Another object of this invention is to provide a PSK communications system which requires a relatively narrow bandwidth channel for its utilization.
A still further object of this invention is to provide a phase modulator for use in a PSK communications system which does not introduce undesirable amplitude modulation to the signal being modulated.